This invention relates to an optical system for shaping the sectional configuration of a light parallel beam.
Emergent rays from a semiconductor laser are different in divergent angle. Therefore, in the case where a bundle of parallel rays is obtained by using a rotationally symmetric collimator lens, emergent rays from the collimator lens are elliptical in intensity distribution.
Therefore, in order to use the laser beam efficiently and to focus it into a spot which is rotationally symmetric in intensity distribution, it is a required to use the afocal anamorphic optical system in which the wavefront aberration has been sufficiently corrected.
Also, in the case of a laser beam circular in section, sometimes it is required to use for a laser beam printer optical system an afocal anamorphic optical system in which for instance the height-width ratio of the laser beam applied to the image forming lens is changed to thereby change resolving powers in horizontal and vertical direction into desired values.
In a conventional afocal anamorphic optical system used for the above-described purposes, two cylindrical lenses are employed, or one or two prisms are used. However, the conventional afocal anamorphic optical system is disadvantageous as follows:
In the afocal anamorphic optical system using two cylindrical lenses, the plane wave is applied to the cylinder surface, and therefore the occurrence of aberration cannot be avoided. Accordingly, in designing an optical system using cylindrical lenses, aberration correcting means are employed to totally negate aberration due to more than one surface as much as possible. However, the afocal anamorphic optical system using a single prism suffers from the difficulty that a large wavefront aberration remains uncorrected if the cylindrical axes of the surfaces to be corrected are even slightly not in alignment with each other. Accordingly the machining and mounting accuracy of the cylindrical lenses requires severe tolerances.
On the other hand, the afocal anamorphic optical system is advantageous in that no aberration takes place because the plane wave is incident on a planar surface. However, in the above optical system, the angle of the bundle of incident rays is deviated from that of the bundle of emergent rays. This will become a disadvantage in designing optical systems.
This disadvantage can be eliminated by a beam shaping optical system which uses two prisms as shown in FIG. 1. In the optical system, of FIG. 1 the bundle of incident rays is parallel to with the bundle of emergent rays. However, the optical system is still disadvantageous in that the system on the incident side which is disposed before the optical system and the system on the emergent side which is disposed after the optical system must be arranged with the optical axes of the two systems from one another in the same plane.